This invention relates to a reaction mass and a spring configured to form a compact and economical internal “oscillator” which is well suited for use in a wave energy converter (WEC) system.
A known class of wave energy converter (WEC) systems includes two bodies [i.e., a “float” (or “shell”) and a “spar” (or “shaft” or “column” or “piston”)] which are designed to move relative to each other and a power-take-off device (PTO) coupled between the two bodies to convert their relative motion into useful energy (e.g., electrical power). A problem with these two body WEC systems is that the bearings and linkages between the float and spar and those associated with the PTO are complex and expensive because of the need to operate in water and because they are subjected to marine growth, contamination and corrosion. Also, the extent to which the float and spar can move relative to each other is limited thereby decreasing the potential for energy collection. Also, the design of a mooring (anchoring) system for a WEC consisting of two or more moving objects that interact directly with the water and waves is often complex.
The problems discussed above are overcome in known WEC systems which include a WEC device contained within a single body (e.g., a “float”) that is acted upon by the waves. The WEC device includes a “reaction mass” attached to a spring and a power take-off device, coupled to the reaction mass. In this type of system, the enclosed reaction mass (m) is suspended from or supported by a mechanical spring that is connected to the float and whose force constant (k) is tuned to give the desired natural period (Tn) of the WEC.
Problems pertaining to the use of conventional mechanical spring systems are discussed in U.S. Pat. No. 7,443,046, issued to Stewart et al, (Stewart being the present applicant) and whose teachings are fully incorporated herein by reference. As noted in U.S. Pat. No. 7,443,046 the prior art (as shown in FIG. 1, hereof) requires a very long spring to achieve a mass-spring oscillation period near that of the dominant wave period. As discussed in the cited patent, it is not practical to construct or house a spring of required length within the float. The length of the spring in still water (x0) can be determined by solving the two following equations simultaneously.m·g=k·x  Equation 1√{square root over (k/m)}=fn=2π/Tn  Equation 2Equation 1 shows that the downward force of the reaction mass (m·g) is equal to the upward force of the spring (k·x) in static conditions. Equation 2 shows that the mass (m) and spring force constant (k) can be selected to give the mass-spring oscillator a natural oscillating frequency near that of the predominant waves.
If the two equations are solved simultaneously, the still-water spring length (x0) would be:x0=(Tn/2π)2·g  Equation 3If the mass-spring system is tuned for a 4-second wave (T), the length of the spring (x0) would be approximately 4 meters. If the mass-spring system is tuned for an 8-second wave (T), the length of the spring (x0) would be approximately 16 meters.
Applicant has previously suggested various systems for reducing the need for physically long springs. One of these includes a WEC device (see prior art FIG. 2 hereof which is a reproduction of FIG. 5 of U.S. Pat. No. 7,443,046) using a lever-based approach coupling the reaction mass and spring. Others include hydraulic-based, or pneumatic-based approaches, which solve the long-spring, long resonant period problem. These WEC systems are well-suited for many applications.
However, they may not be best suited for small and medium sized floats which require a more compact and more economical “oscillator” configuration.